In case a fuel cell system is applied to a movable body such as a vehicle, a liquid medium such as purified water used for cooling or humidification for a fuel cell is used to be circulated. When the liquid medium is, however, used and circulated over the long term, due to ion separation from a casing of the fuel cell or eluting materials from various hoses or pipes, density of impurities in the liquid medium is gradually increased, which tends to introduce clog or corrosion of the pipes. As an apparatus to remove these impurities, there is in general used an ion exchange filter filled with an ion exchange resin.
Japanese Patent Publication No. 2001-35519A has disclosed that an ion exchange filter is used for removing impurities in a medium and further, a circuit to bypass the ion exchange filter is disposed to perform ion-exchange treatment only to a part of the medium for downsizing of the ion exchange filter.